Process for thermoforming reinforced polymer sheets

ABSTRACT

Reinforced poly(ethylene terephthalate) sheets are thermoformed to articles having one or more surfaces in contact with the mold of such a degree of smoothness that they virtually completely replicate the mold face. When the mold face is completely smooth, the concave imperfections caused by air entrapment in the plastic material account for less than 6% of the surface area. Composite articles consisting of a face member and a backing member are made by thermoforming in a single mold a face member having one or more surfaces in contact with the mold face of such a degree of smoothness that it completely replicates the mold face, and its proportion of concave imperfections is less than about 6% of the surface area. The backing member is bonded in the mold with an adhesive to the face member, and the composite structure is allowed to remain in the mold for a sufficient time to permit subsequent removal without distortion. The initial polymer sheets are less than about 5% crystalline, but the thermoformed article is left in the mold until it crystallizes to the desired degree.

CROSS-REFERENCE TO RELATED APPLICATION

This is a contination-in-part of our application Ser. No. 355,711 filedMar. 8, 1982, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a process for thermoforming reinforced polymersheets and forming composite structures wherein at least one exteriorsurface exactly replicates the mold surface.

Thermoformed articles made of thermoplastic polymer sheets findapplication, among others, in the automotivie industry as well as inother industries. Because plastic offers considerable weight savingsover metal, it has been increasingly used for making various auto partsthat previously were stamped out of steel sheets. However, plastic bodypanels have not found wide acceptance, mainly because of theirunsatisfactory appearance, which is due in part to defects caused by airtrapped between the starting plastic sheet and the adjacent moldsurface. When the sheet is very soft, it tends to collapse onto the moldsurface before all the air can be removed, and the resulting air pocketscause imperfections on the sheet surface. This is especially so forshallow draw thermoforming. When the sheet is too hard, it cannotexactly replicate the mold surface and appears fairly rough. Thesedefects are exaggerated when the thermoformed part is painted. See, forexample, the article entitled "Detroit moves closer to plastic bodypanels" in the March, 1979, issue of Business Week, pp. 84F and 84G, thenote entitled "Cosmetic repair of SMC produces Class A finish" on page74 of the April, 1979, issued of Plastics World, and the discussion ofsurface quality on p. 74 of the November, 1980, issue of Plastics World.In all these texts it is suggested that surface imperfections can becorrected by subsequent coating.

A further problem is encountered when the thermoformed sheet is adheredto a backing sheet, which may serve as a reinforcing member and may beribbed or corrugated for greater strength and rigidity. Frequently thepoints or areas of bonding of the backing sheet can be seen when lookingat the outer face of the front sheet. This phenomenon, known as the"print through" or "read through", can be caused by the practice ofeffecting the bonding at a temperature at which both members are at orabove their glass transition temperatures and are not crystallized.Print through can be avoided by use of a suitable adhesive, which canprovide at moderate temperatures composites having good structuralintegrity.

It thus is desirable to be able to thermoform reinforced poly(ethyleneterephthalate) (PET) sheets into articles which have at least one smoothsurface, exactly replicating the mold surface, without imperfections,and to form composites which, in addition to having at least one suchsurface, do not suffer from the print through effect.

SUMMARY OF THE INVENTION

According to the present invention, there is now provided in a processfor thermoforming a poly(ethylene terephthalate) sheet having acrystallinity of about 0-5% and a thickness of about 0.76-3.2 mm,preheated to a temperature of about 80°-130° C., at a draw ratio of lessthan about 1.8, defined as the ratio of the initial sheet thickness tothe minimum wall thickness of the thermoformed article, and at apressure differential of about 200-2000 kPa, in a mold having at leastone surface adjacent of said sheet of average roughness R_(A), of about0.1-0.8 μm,

the improvement, which produces a thermoformed article in which itsessentially entire surface in contact with said mold surface has anR_(A) of about 0.1 to 0.8 μm and the concave optical imperfection indexof said surface is less than about 6% of the surface area, as determinedby image analysis using a microscope having a magnification of 70 times,comprising the following steps:

(A) removing air from the space between said mold surface and saidthermoplastic sheet through at least one vent in said mold surface topermit complete contact of said mold surface with said sheet,

(B) while applying air pressure to the backside of said sheet, bringingsaid sheet, before its poly(ethylene terephthalate) crystallinityexceeds about 15%, in complete contact with said mold surface maintainedat a temperature of about 130°-180° C., to permit the sheet polymer tocompletely replicate said mold surface,

(C) maintaining said sheet in contact with the mold at a temperature of130°-180° C. at least until the sheet polymer reaches a degree ofcrystallinity sufficient to permit demolding without distortion, and

(D) demolding to thermoformed article;

with the proviso that the poly(ethylene terephthalate) sheet isreinforced with a mineral filler and, when so reinforced, has a tensilemodulus at 100° C., determined as an average of machine direction andtransverse direction, within the range of about 7 to 70 MPa, asdetermined according to ASTM D-638.

Additionally, there also is provided in a process for thermoforming in amold a first sheet of poly(ethylene terephthalate) having acrystallinity of about 0-5% and a thickness of about 0.76-3.2 mm,preheated to a temperature of about 80°-130° C. at a draw ratio of lessthan about 1.8, defined as the ratio of the initial sheet thickness tothe minimum wall thickness of the thermoformed article, the surface ofthe mold adjacent said sheet having an average roughness, R_(A), ofabout 0.1 to 0.8 μm, and a second, reinforcing, poly(ethyleneterephthalate) backing sheet and bonding the second sheet to the firstwhile both sheets are in the mold,

the improvement, which results in the first sheet having its essentiallyentire unbonded surface of an R_(A) of about 0.1 to 0.8 μm, the concaveoptical imperfection index of said surface being less than about 6% ofthe surface area, as determined by image analysis using a microscopehaving a magnification of 70 times, said improvement comprising thefollowing steps:

(A) removing simultaneously or sequentially air from the spaces betweenboth sheets and their adjacent mold surfaces, maintained at atemperature of about 130°-180° C., through at least one vent in eachmold surface,

(B) while applying air pressure between both sheets, forming both sheetsinto their desired shapes while the first sheet has a poly(ethyleneterephthalate) crystallinity of at most about 15%, to permit the firstsheet polymer to completely replicate the mold surface,

(C) maintaining both sheets in contact with the hot mold surfaces atleast until the polymer in each sheet polymer reaches a degree ofcrystallinity sufficient to permit bonding without distortion,

(D) bonding the second sheet to the first with an adhesive, while thefirst sheet is in contact with the mold,

(E) maintaining the first sheet in contact with the mold until thebonded structure can be demolded without distortion, and

(F) demolding the bonded article;

with the proviso that the first sheet is reinforced with a mineralfiller and, when so reinforced, has a tensile modulus at 100° C.,determined as an average of machine direction and transverse direction,within the range of about 7 to 70 MPa, as determined according to ASTMD-638.

DETAILED DESCRIPTION OF THE INVENTION

A PET sheet having a crystallinity of about 0-5% is considered amorphousfor all practical purposes. Crystallinity is determined from the sheet'sdensity, as explained later in this disclosure. An amorphous PET sheet,when heated to a temperature of about 80°-130° C., will remainsubstantially amorphous for a period of several seconds to severalminutes, but then it will begin to substantially crystallize. Preheatingshould be uniform to avoid localized hot spots and preferably should bedone quickly.

Thermoforming articles from polymer sheets is well known. A polymersheet is heated above its softening temperature, placed in a hot mold,and brought in contact with mold walls by the use of vacuum, pressure ora combination of both. Typical thermoforming processes and equipment aredescribed, for example, in U.S. Pat. Nos. 3,935,358 and 3,982,877, bothto N. C. Wyeth et al. In the case of reinforced PET, according to thepresent invention, it is important that all air be removed from betweenthe mold face which is to be exactly replicated and the PET sheet. Thismay be accomplished either by the use of vacuum prior to orsimultaneously with the application of air pressure to the back side ofthe sheet or simply by the use of air pressure alone. Usually, bothvacuum and air pressure will be employed for a thermoformed surface ofsatisfactory quality. The optimum pressure differential depends amongothers on the tensile modulus of the filled PET sheet, so that therequired pressure differential increases with the modulus but notnecessarily in a straight relationship. As a rule of thumb, a pressuredifferential ##EQU1## will always give satisfactory results, butthermoformed articles of the desired quality may also sometimes beobtained at a lower pressure differential. Whenever measurements aremade or results are obtained in units other than SI (metric), theyshould be converted to SI units. Thus, kPa stands for kilopascals andMPa for megapascals.

The mineral filler can be any material which is effective as areinforcing agent. Most such materials will be in the form of fibers,for example, glass fibers, graphite fibers, and quartz fibers; othersare in the form of platelets, for example, various types of mica; stillothers may be granular, for example, glass spheres, carbon black,calcium sulfate, and titanium dioxide. A filler is an effectivereinforcing agent when the filled composition has a higher tensilestrength and tensile modulus than the unfilled base polymer. The levelof reinforcing filler will vary but normally will be about 5-50% byweight of the total composition, preferably 10-35%. The preferred filleris glass fibers, especially those having an average length-to-diameterratio of at least 10. In general, fillers which have an aspect ratio of10-35 are very effective reinforcing materials. The aspect ratio is theratio of the largest to the smaller dimension.

The reinforcing material imparts to the filled composition sufficientstiffness at the thermoforming temperature to permit a virtuallycomplete elimination of air from the space between the mold surface andthe sheet before full contact of the sheet with the mold is made,especially in shallow draw molds, where the ratio of the initial sheetthickness to the minimum wall thickness of the formed article is 1.3 orless. Full contact in this context does not mean merely conforming theshape of the sheet to the shape of the mold but having at least 94% ofall points on the sheet surface in contact with the mold surface so thatevery detail of the mold surface is reproduced on the sheet, and theconcave optical imperfection index is below 6% of the area. The concaveoptical imperfection index is an experimental value which indicates whatproportion of the total surface area is occupied by micropores due toair entrapment. It is best measured by Quantitative Image Analysis usingQMS equipment of Bausch & Lomb, which provides a fast and accurate scaninterpretation. In practice, twenty randomly selected rectangularsurface areas, 1.4× 1.9 mm, are scanned, and the average value iscalculated. Because the sheet material must completely wet the moldsurface, it is important to bring the sheet and the mold in contactwhile the sheet polymer is still substantially amorphous and soft; thatis, before substantial crystallization occurs. This requirement limitsthe maximum draw ratio because at draw ratios higher than about 1.8stress-induced crystallinity prevents adequate polymer flow for completesurface contact. Contact of the sheet with the mold is then maintaineduntil crystallization has reached a sufficiently high level to permitdemolding without deformation.

Although the inventors do not wish to be bound by any specificscientific theory, it appears that the success of this invention dependsto a large extent on the ability of the polymer in the PET sheet matrixto flow through the filler material from the backside to the front side,so that a thermoformed sheet may be expected to contain a larger amountof polymer material on its front side. Certain analytical techniqueslend support to this theory; they are: attenuated total reflectance(ATR), which relies on the relative proportions of reflected andabsorbed radiation; X-ray diffraction, which is based on the fact thatthe intensity of the diffraction pattern depends on the concentration ofthe crystalline phase in the total multiphase composition; and electronspectroscopy chemical analysis (ESCA), which permits to accuratelydetermine the silicon content on each side of a glass fiber-filled PETsheet. The silicon concentration, naturally, is proportional to theglass content.

It has been often found advisable to further modify the composition byadding to the PET another polymeric material, namely, either a dipolymerof ethylene with an ethylenically unsaturated carboxylic acid or aterpolymer of ethylene with an unsaturated carboxylic acid and with anester of an ethylenically unsaturated carboxylic acid or with a vinylester. The proporation of the carboxylic acid in the dipolymer is about2-20 weight percent and the respective proportions of the carboxylicacid and of the ester in the terpolymer are about 1-30 weight percent ofeach, and the amount of the dipolymer or terpolymer is about 1-20% basedon the weight of the final, reinforced polymer composition. Thecarboxylic groups of the dipolymer or terpolymer can either be free orcan be converted to their salt form, the cation being a divalent metalion. Suitable metal ions are, among others, calcium, zinc, andmagnesium. Such partially or completely neutralized carboxylatedpolymers are well known to the art under the generic name "ionomers38 .Preparation of such materials is described, among others, in U.S. Pat.Nos. 3,264,272 to Rees and 3,639,527 to Brinkmann et al., and someionomers are commercially available. Typical unsaturated carboxylicacids include: acrylic, methacrylic, maleic, and fumaric acids,3-butenoic and 4-pentenoic acids, and itaconic and aconitic acids.Typical esters include, for example, metbhyl acrylate, ethylmethacrylate, isobutyl acrylate, diethyl fumarate, dimethyl maleate, andvinyl acetate. The preferred amount of the carboxylic acid copolymer orterpolymer is about 3-10 weight % of the reinforced polymer composition.

Representative tensile modulus values at 100° C. for glassfiber-reinforced PET sheets are given below. In all cases, thecrystallinity of the test samples was below about 5% at the time themeasurements were made. At higher crystalinities the moduli would behigher.

    ______________________________________                                        Composition, Wt. %                                                                            100° C. Average                                        Glass/Ionomer/PET                                                                             Tensile Modulus, MPa                                          ______________________________________                                         0       0      100      1.72                                                 29       6      65      21.7                                                  32       6      62      30.1                                                  36       6      58      34.1                                                  20       0      80      25.1                                                  32       0      68      38.3                                                  ______________________________________                                    

The values of tensile modulus depend to some extent on the methodemployed to prepare the filled PET compositions. For example,compositions made in a twin-screw extruder (like those above) can beexpected to have lower tensile moduli than those made in a single-screwextruder since more breakage of the filler material (especially glassfiber) occurs in the former than in the latter.

Thermoforming a reinforced PET sheet can be readily accomplished inconventional equipment, for example, such as shown in FIGS. 1-5. FIG. 1is a side view of a horizontal thermoformino mold, which may be mountedin a benchtop hydraulic press having a stationary platen 4 and a movableplaten 3. Platen 4 carries mold member 8, and platen 3 carries moldmember 7. Channel 10 drilled through member 8 and communicating withcentrally located channel 10a is connected with a source of compressedair (not shown). A similar channel 11 drilled through member 7 andcommunicating with a centrally located channel 11a is connected with asource of vacuum (not shown). Mold member 7 carries spherical mold plate13 which is held in place by clamp ring 9. Clamp ring 9 has around itscircumference ridge 29, which is aligned with circular slot 16 in moldmember 8 to provide a perimeter seal during the operation. Mold plate 13has on its backside slot ring 25, which is intersected by small holes 23regularly distributed around the perimeter of mold plate 13. Holes 23,which are connected to vacuum channel 11 by means of slot ring 25 andpassage 27, serve to apply vacuum to the face of mold plate 13.

Each mold member is heated by electric heating rods, which have separatecontrols and can be maintained at different temperatures within therange of about 130°-180° C.

FIG. 2 is the front view of the same thermoforming mold. This figureshows, among others, heating rods 18 and 31, compressed air channels 10and 10a, vacuum channels 11 and 11a, and thermocouples 20 and 33.

In operation, reinforced poly(ethylene terephthalate) sheet 35 ispreheated in a flat press to a temperature of about 80°-130° C., thentransferred to the thermoforming mold, as shown in FIG. 3. Preheatingcan be effected, if desired, in other types of heating equipment or evenin the thermoforming mold itself. Mold member 7 is then raisedhydraulically into contact with mold member 8. As shown in FIG. 4, thiscloses clamping ring 9 against mold member 8 to effect a perimeter seal.Mold member 7 is then evacuated via channel 11. This causes sheet 35 toconform generally, but not completely, to the contour of mold member 7and particularly mold plate 13. Pressure is now applied through channels10 and 10a. Sheet 35 is thus forced against the face of mold member 7,and particularly of mold plate 13, and acquires its final shape,conforming to mold plate 13, as shown in FIG. 5. The pressure is thenrelieved, and the sheet is left in the mold until the desired degree ofcrystallinity has been reached. The vacuum is relieved, and the mold isopened to remove the thermoformed article.

The starting reinforced PET sheet normally has a rough surface and,depending on the amount of mineral reinforcing agent, can be quitestiff. In order to thermoform an article having a very smooth surface,it is necessary to maintain the right temperature and pressureconditions not only to permit the sheet to conform to the shape of themold but also to permit the molten polymer material to flow through thereinforcing agent so as to make an essentially complete contact with themold.

Several experiments were run to study the various parameters affectingthe operability of the single sheet thermoforming process according tothe present invention. Glass fiber-reinforced poly(ethyleneterephthalate) sheeting, optionally also containing Du Pont "Surlyn"1855 ionomer, was used for this purpose. For comparison, unreinforcedPET also was tested. In all cases, PET sheeting was extruded in aconventional twin-screw extruder maintained at a barrel temperature of265°-270° C. through a die heated to 275° C. and taken up on a standardthree-roll finisher maintained at top, middle, and bottom rolltemperatures of 25, 52, and 25° C. respectively. The sheeting had anaverage thickness of 60 mils (1.524 mm) and had a crystallinity of lessthan 5%, as determined by its density measurements. Poly(ethyleneterephthalate) was Goodyear "Cleartuf" 7202A resin having inherentviscosity of 0.72 dL/g, as determined at 25° C. at a concentration of0.32 g/100 mL in a mixture of 25 vol. % of trifluloroacetic acid and 75vol. % of methylene chloride, and glass fibers were Owens-Corning -Fiberglass 416×15 choppes strands, 3/16 inch (4.8 mm) long.

Sheets, 6×6 inches (15.24×15.24 cm) were cut from the sheeting and usedin the tests. The surface roughness of both sides of the sheets wasdetermined with a profilometer, both in the machine direction (MD) andtransverse direction (TD). The profilometer used in these experimentstravels a distance of 2.5 cm across the surface of the sheet andgenerates an average roughness number, Ra, in either microinches ormicrometers. The surface of a sheet must be flat during the measurement.

The thermoforming mold corresponds to that represented in FIGS. 1-5,wherein mold plate 13 has a diameter of 4.1 inches (10.25 cm), and clampring 9 has the effective height (excluding ridge 29) of 0.28 cm. Thesurface of mold plate 13 has an average roughness, Ra, of 0.016 inch(0.4 μm). The mold plate material is aluminum 6061. The mold temperaturewas 140° C. The following parmeters were studied: sheet composition,preheat temperature and time, and thermoforming vacuum and pressure. Theresults of these tests are provided in the following Table 1. In allcases where both vacuum and pressure were applied, vacuum was appliedfirst, and pressure was applied 3 seconds later, except in Example 15,where vacuum was applied 9 seconds later. In all cases, the draw ratio,determined according to the definition in this disclosure, was 1.05.

                  TABLE 1                                                         ______________________________________                                                 Composition Preheat                                                  Sample No. glass/ionomer/PET                                                                           T, °C.                                                                             t, sec.                                  ______________________________________                                        1          0      0      100   25        --                                   2          0      0      100   60        10                                   3          0      0      100   80        20                                   4          0      0      100   100       20                                   5          0      0      100   110       20                                   6          0      0      100   130       10                                   7          0      0      100   140       10                                   8          0      0      100   110       20                                   9          0      0      100   110       20                                   10         0      0      100   110       20                                   11         0      0      100   110       20                                   12         29     6      65    25        --                                   13         29     6      65    110       15                                   14         29     6      65    140       15                                   15         29     6      65    110       15                                   16         29     6      65    110       15                                   17         29     6      65    110       15                                   18         29     6      65    110       15                                   19         29     6      65    110       15                                   20         29     6      65    110       15                                   21         29     6      65    110       15                                   22         32     6      62    110       15                                   23         32     6      62    110       15                                   24         32     6      62    110       15                                   25         36     6      58    110       15                                   26         20     0      80    25        --                                   27         20     0      80    110       15                                   28         32     0      68    25        --                                   29         32     0      68    110       15                                   ______________________________________                                               Vacuum     Pressure   COII*                                            Sample No.                                                                             Inch. Hg kPa     psi  kPa   % Area                                   ______________________________________                                        1        30       101     100  690          **                                2        30       101     100  690          **                                3        30       101     100  690          **                                4        30       101     100  690          **                                5        30       101     100  690          **                                6        30       101     100  690          **                                7        30       101     100  690          **                                8        30       101      60  414          **                                9        20        67     100  690          **                                10       --       --      100  690          **                                11       30       101     --   --           **                                12       30       101     100  690   1.93                                     13       30       101     100  690   2.29                                     14       30       101     100  690   32.3                                     15       30       101     100  690   4.76                                     16       10        33     100  690   2.87                                     17       15        50     100  690   3.52                                     18       30       101      80  552   2.40                                     19       30       101      40  276   8.27                                     20       --       --      100  690   5.81                                     21       30       101     --   --    57.8                                     22       30       101     100  690   1.76                                     23       15        50     100  690   4.14                                     24       30       101      80  552   7.24                                     25       30       101     100  690   9.17                                     26       30       101     100  690   1.53                                     27       30       101     100  690   2.88                                     28       30       101     100  690   1.89                                     29       30       101     100  690   1.17                                     ______________________________________                                         *COII: Concave Optical Imperfection                                           **Large areas of surface retained initial sheet                          

roughness because entrapped air prevented complete contact of the sheetwith the mold. As a result, these samples had gross imperfections 1 mmto several centimeters in size, which were immediately apparent to theeye.

As can be seen from the above table, all unreinforced PET sheets gaveunsatisfactory results. Reinforced PET sheets were thermoformed to afinished product having low surface roughness and low concave opticalimperfection index (COII). Surface roughness data are given below inTable 2.

                  TABLE 2                                                         ______________________________________                                                  Cosmetic Side*                                                                             Back Side                                              Sample No.  MD      TD         MD   TD                                        ______________________________________                                                  Initial Surface, Ra μm                                           1           0.33    0.43       0.30 0.43                                      2           0.33    0.28       0.36 0.30                                      3           0.38    0.41       0.41 0.36                                      4           0.51    0.46       0.43 0.30                                      5           0.51    0.64       0.51 0.25                                      6           0.48    0.41       0.33 0.38                                      7           0.61    0.51       0.30 0.30                                      8           0.51    0.46       0.25 0.30                                      9           0.56    0.43       0.33 0.25                                      10          0.56    0.46       0.36 0.36                                      11          0.46    0.30       0.30 0.36                                      12          4.32    5.33       4.57 4.57                                      13          3.81    4.32       4.57 4.57                                      14          4.32    4.06       5.08 4.83                                      15          4.32    4.32       5.08 5.33                                      16          5.08    5.33       5.33 5.59                                      17          4.06    4.32       5.08 5.33                                      18          5.08    5.33       5.33 5.33                                      19          3.81    4.06       4.83 5.33                                      20          5.08    5.59       5.08 4.83                                      21          4.32    3.81       5.08 4.57                                      22          5.84    6.10       6.10 6.10                                      23          6.10    6.10       4.57 4.83                                      24          6.35    6.86       5.59 6.35                                      25          5.59    6.10       5.33 5.08                                      26          4.06    4.06       1.78 2.03                                      27          5.08    5.33       1.78 2.29                                      28          7.11    8.13       6.35 5.84                                      29          6.60    6.60       5.84 5.59                                                Final Surface, Ra μm                                             1           0.20    0.23       0.18 0.20                                      2           0.30    0.33       0.25 0.43                                      3           0.23    0.20       0.20 0.15                                      4           0.30    0.30       0.20 0.20                                      5           0.33    0.36       0.38 0.25                                      6           0.25    0.36       0.46 0.46                                      7           0.36    0.28       0.64 0.64                                      8           0.23    0.23       0.46 0.43                                      9           0.28    0.41       0.46 0.51                                      10          0.28    0.30       0.71 0.76                                      11          0.33    0.46       0.64 0.56                                      12          0.38    0.33       4.57 5.33                                      13          0.33    0.36       4.83 4.06                                      14          0.33    0.38       1.27 1.14                                      15          0.43    0.56       4.57 4.06                                      16          0.41    0.41       3.81 4.06                                      17          0.38    0.36       4.57 4.83                                      18          0.36    0.33       3.56 3.56                                      19          0.46    0.58       3.81 3.56                                      20          0.41    0.76       3.56 3.30                                      21          2.54    2.29       4.06 3.81                                      22          0.36    0.33       2.79 3.30                                      23          0.46    0.38       4.06 4.83                                      24          0.56    0.61       3.81 5.08                                      25          0.64    0.66       5.08 4.57                                      26          0.33    0.33       4.83 5.08                                      27          0.36    0.38       2.79 3.56                                      28          0.41    0.41       5.59 5.33                                      29          0.46    0.51       5.08 5.08                                      ______________________________________                                         *"Cosmetic side"is the side in contact with mold face 13 (FIGS. 1-5), for     which a low COII and a low Ra is desired.                                

At the mold temperature, the crystallinity of the PET sheet increaseswith time until it reaches a plateau.

The degree of crystallinity of crystallized, reinforced plastic sheet isdetermined from density measurements as follows:

First, sheet density is calculated from the following equation:

    D.sub.2 =X.sub.2 /(1/D.sub.s -X.sub.1 /D.sub.1)

where

D₂ =polymer density

X₂ =polymer weight fraction

D_(s) =Sample composite sheet density

D₁ =Reinforcing agent density

X₁ =Reinforcing agent weight fraction

The density of the composite sheet is determined by measuring theapparent loss of weight while the sheet is immersed in deaerated,distilled water and using the following equation:

    D.sub.s =D.sub.H (WA/(WA-WH)

where

D_(H) =Density of water taken as 1.0

WA=Sample weight in air

WH=Sample weight in water

The percent crystallinity is determined using the following equation:

    ______________________________________                                        Percent Crystallinity =                                                                       [(D.sub.2 -DA)/(DC - DA)] × 100                         where DA =     Density of amorphous polymer                                   DC =           Density of 100% crystalline                                                   polymer                                                        For PET DA =   1.333 g/cm.sup.3 and                                           DC =           1.455 g/cm.sup.3                                               ______________________________________                                    

FIG. 6 is a graph representing the relationship between the PET sheetcrystallinity and the time in seconds, from the closing of the mold(clamping ring 9 closes against mold member 8, as shown in FIG. 4). Itcan be seen that the crystallinity of the PET sheet at the moldtemperature of 140° C. remains at not over about 15% during the first 6seconds. According to the operating procedure in these experiments, thesheet was fully in contact with the mold surface within 3-5 seconds, sothat the crystallinity condition at the time of contact was satisfied.This curve is considered to be only typical and not general since itsactual shape will depend in each case on the preheat history of thesample. However, it is quite representative of this importantrelationship.

It often is desired to bond to the backside of a thermoformed plasticsheet a reinforcing plastic member (frequently ribbed or corrugated forgreater strength), which may be formed in the same operation as thefirst sheet. It has been found most practical to use adhesive bonding toavoid print through. The face member and the backing member must firstbe allowed to harden by crystallization sufficiently to permit adhesivebonding to each other without deformation or print through.

FIG. 7 is the side view of a thermoforming apparatus useful in formingcomposite articles according to the process of this invention. Itcomprises frame 40 having end plates 43 and 44, four guide rods 41secured to end plates 43 and 44, a stationary platen 49, and a movableplaten 46 supported and guided by rods 41. Platen 46 is connected to rod47 of hydraulic cylinder 48 mounted on end plate 43 and can thus behydraulically moved from left to right. Platen 46 carries mold member55. Platen 49 carries mounting plate 45 on which is mounted mold member56. Each mold member is heated by hot oil circulating through internalpassages not shown. The rate of flow of hot oil can be controlled byseparate valves (not shown), so that each mold member can be maintainedat a different temperature, if desired. At the beginning of theoperating cycle, movable platen 46 is brough into a position (shown inbroken lines) where the faces of mold members 55 and 56 are about 10 cmapart.

FIG. 8 is the end view of the apparatus shown in FIG. 7. Referring nowto FIG. 8, 53 is a sheet transporter, which comprises a rodless aircylinder 54 hung from guide rods 41 and a hanging means, 57, forsupporting plastic sheets. It extends from sheet heating position 65 toloading/unloading position 66.

The hanging means 57 has a fluid passage (not shown), which is suppliedwith compressed air through hose 61, as shown in FIG. 8. Plastic sheets60 are hung in pairs in hanging means 57 and moved to the heat zone.Sheet heating means 65 consists of two electrical radiant heaters, whichcan be maintained at the same or different temperatures. Plastic sheets60 are moved into this space and held there for the desired time, e.g.,5-60 seconds. The sheet which is to become the face member of the finalthermoformed article is thus preheated to about 80° C. and the sheetwhich is to become the backing member is preheated to a convenienttemperature, for example, 80°-100° C. As in the case of single sheetthermoforming, above, other preheating means, including thethermoforming mold itself, can be used. The sheet transporter thencarries the sheets into the space between mold members 55 and 56.

FIG. 9 shows the initial arrangement of mold members 55 and 56 seen fromthe same side as in FIG. 7. Mold member 55 is intended to form thesmooth (cosmetic) face member of the composite article, while moldmember 56 forms the backing, reinforcing member. Extending around threesides of mold member 55 is pinch bar 82, which has a V-shaped ridge 82aaligned with slot 38 on mold member 56. Extending across the tops ofmold members 55 and 56 are knife edges 84. The function of the pinch barand knife edges is to form a perimeter seal. However, the seal at thetop formed by knife edges 84 still allows air to be introduced betweenthe two plastic sheets 60. Mold plate 62 has on its backside slots 71,which are intersected by small diameter holes 72 regularly distributedaround the perimeter of plate 62. Holes 72 emerge on the outer face inclose proximity to the inner surface of clamp frame 39 and serve toapply vacuum to the face of plate 62.

Inside the cavity in mold member 55 is slot 73 which communicates withslots 71 in mold plate 62 and also connects to drilled holes 88, one ofwhich extends to an outer surface of mold 55, where it is connected tovacuum (not shown). Mold face 86 of mold member 55 is finished to anaverage surface roughness, R_(A), of about 0.1 to 0.8 μm.

Mold member 56 has a rectangular cavity occupied by plate 93, which isslightly smaller than the cavity and leaves slot 52 on all sides. Behindslot 52 is a larger slot 59 which runs around the entire perimeter ofthe mold cavity and communicates with holes 51, one of whichcommunicates with a source of vacuum (not shown). Plate 93 may be moveda small distance to the left by means of pistons 50 (one shown).Compressed air can be admitted to pistons 50 via drilled channel 75.Pyramidical members 74 attached to plate 93 serve to producecorrugations on the plastic sheet being thermoformed.

In operation, a sheet 100 of reinforced PET and a backing sheet 101 areassembled as shown in FIGS. 10 and 11 and hung from hanging means 57 ofthe thermoforming apparatus shown in FIGS. 7 through 9. In FIGS. 10 and11 the face sheet 100 is smooth and the backing sheet 101 is ribbed toprovide air passages. Prior to the assembly, the inside surfaces ofeither one or both sheets are coated with a suitable adhesive.

With reference to FIG. 8, sheets 60 initially are in position 66, fromwhich they are moved by means of sheet transporter 53 through the openmold behind platen 46 to heating means 65. After a preheating cycle of20 seconds, the assembly is moved into the open mold space, as shown inFIG. 9. Mold member 55 is then moved right into contact with mold member56. As shown in FIG. 12, pinch bar edge 82a and knife edges 84 effect aperimeter seal. Mold member 55 is evacuated, causing both sheets 60 toconform generally, but not completely, to the contour of this moldmember. Compressed air is now introduced between sheets 60 through themanifold in hanging means 57, and vacuum is simultaneously applied tomold member 56. As shown in FIG. 13, both sheets 60 are now forcedagainst the faces of their respective mold members and acquire theirfinal shapes conforming to those of the mold plates. The face sheetstill is in a substantially amorphous state, below a crystallinity ofabout 15%, and, because of its intimate contact with the mold plate, itcompletely replicates the smooth finish of the mold plate, so that itsdegree of roughness, R_(A), is essentially the same as that of the mold.Continued contact of both sheets with the hot mold members promotescrystallization of the polymer. When the desired degree of crystallinityhas been reached, air pressure between the sheets is relieved. Airpressure is then applied to pistons 50 in mold member 56. This drivesplates 93 to the left, so that the apices of the pyramids 74 bring thecorrugations of the backing sheet in contact with the face sheet,producing bonds at contact points as shown in FIG. 14. Alternatively,pistons 50 can drive plate 93 to the left while pressure is maintainedbetween the plastic sheets. The bonded assembly is maintained in themold for sufficient time to achieve sufficient hot bond strength. Airpressure on piston 50 is then relieved and vacuum is applied to channel75, causing plate 93 to withdraw to its initial position. Platen 46 isretracted by means of cylinder 48 and rod 47, withdrawing mold member55.

The thermoformed, molded assembly is now moved to the unloading area 66shown in FIG. 8 and removed from the hanging means.

In order to avoid distortion of the thermoformed composite structure, itis important to have both the face member and the backing member formedof sheets having matched compositions. While the exact types and amountsof filler materials will not necessarily be the same, they usually willbe close; and when the filler material is oriented, e.g. fibrous, theinitial orientation of both sheets (machine direction of transversedirection) also will be the same. Since the appearance of the backingmember is of considerably less importance than that of the face member,the strict operating parameters that apply to the formation of the facemember may not be necessary with respect to the backing member. Inparticular, depending on the exact composition of the backing member PETsheet, it may be possible to preheat the sheet to a differenttemperature, use a different pressure differential (e.g., by controllingthe amount of vacuum, if any, applied to the back side), and even to usea mold member which will produce a different draw ratio, so long as themechanical integrity of the backing member is not adversely affected.

Since composite articles made according to this invention areadhesive-bonded, rather than melt-bonded, the choice of a suitableadhesive is important. Suitable adhesives must be able to developsufficient bond strength at the thermoforming temperature, which may beas high as 200° C. or even higher, to permit demolding withoutdistortion. The adhesives may be either thermoplastic or thermosetting.They must adhere well to both plastic sheets and should not have a highmodulus when cured because high modulus could cause distortion of theface sheet on cooling. Typical bonding compositions that can be used inthe process of this invention include, for example, epoxy resins,polyurethanes, and amorphous polyesters.

Suitable epoxy adhesives include "Scotch-weld" brand structural adhesive2216 B/A of 3M Company. A good polyurethane adhesive is "UR" 2101two-part urethane of H. B. Fuller Company, and suitable amorphouspolyesters include Eastman's "PETG" copolymer.

In actual experiments using the above-described equipment illustrated inFIGS. 7-9, 12, and 13, composite structures were formed from PET sheetshaving the following composition (wt. %):

    ______________________________________                                                PET     65                                                                    ionomer  5                                                                    glass fiber                                                                           30                                                            ______________________________________                                    

Both sheets had an initial crystallinity of less than about 5% and anaverage tensile modulus at 100° C. of 36.9 MPa.

The face sheet was flat, while the backing sheet was corrugated(ribbed), as shown in FIG. 11.

Each sheet was coated prior to the thermoforming operation withEastman's "PETG" adhesive copolymer, which is solid at room temperaturebut has a glass transition temperature of about 81° C.

The sheets were assembled as shown in FIGS. 10 and 11 so that theadhesive-coated surfaces faced each other. The initial surface roughnessof the face sheet on the side adjacent the mold face (thus, the uncoatedside), Ra was usually about 250 μin. (6.3 μm) but in no case more than500 μin. (12.5 μm). Both sheets were preheated to about 80° C., placedin the open mold, and equilibrated for about 6 seconds before the moldwas closed. The mold temperature was about 130°-150° C., with bestresults obtained at the lower end of this range. The average variationof the mold surface temperature was kept within 3.8° C. The surfaceroughness Ra of mold face 86 of mold member 55 (FIGS. 9, 12, and 13) was16-32 μin. (0.4-0.8 μm).

After mold members 55 and 56 closed and clamped, a vacuum of no lessthan 25 in Hg (84 kPa) was applied to each mold member and held for 1-4seconds. Compressed air then was introduced into the space between thesheets. The air pressure was 85-100 psi (586-690 kPa). Higher pressuresproduced at most a marginal quality improvement. Depending on the PETsheet composition, pressures of less than the lower limit of this rangemay produce surfaces with discernible imperfections.

Vacuum and pressure were held for approximately 90 seconds or until theface sheet reached adequate crystallinity. Compressed air was thenvented, and the vacuum on the back side of the backing sheet wasreleased.

The backing member was moved a distance of 0.2-0.6 in (0.5-1.5 cm) intocontact with the face member. The contact point pressure was 12-75 psi(83-517 kPa). Contact was maintained for about 30 seconds, and thebacking mold plate was withdrawn. Vacuum was then released, and the moldwas opened for composite part's removal.

The average roughness of the cosmetic face of the face member of thecomposite was 16-32 μin (0.4-0.8 μm), the same as that of the mold face,and the concave optical imperfection index (COII) was below 6%.

Articles therformfed by the process of the present invention, whethersingle sheet or a composite structure having a face member and a backingmember, find use in a number of applications, particularly in theautomobile industry. The thermoformed articles thus include, forexample, a variety of automotive parts, especially body panels, bothinterior and exterior, dashboards, floorboards, air dams, headliners,window decks, hoods, truck lids, seats, and underbody parts.Non-automotive applications of the process of the present inventioninclude, among others, the thermoforming of aircraft parts such s bodypanels, seats, and ducts; shipping containers; construction elementssuch as siding panels, overhead garage door panels, and partitions;marine parts such as boat body panels, seating, and deck surfaces;appliance parts including body panels, ducts, and small appliancehousings; institutional food service trays; sanitary ware such as toiletseats, towel dispensers, and shower enclosures; solar collectors;enclosures and housings for a variety of equipment, including electronicequipment, pumps, X-ray machines, and lawnmowers; furniture such aschairs and tables; and a host of other industrial, residential,recreational and transportation uses.

We claim:
 1. In a process for thermoforming a poly(ethyleneterephthalate) sheet having a crystallinity of about 0-5% and athickness of about 0.76-3.2 mm, which is reinforced with a mineralfiller and, when so reinforced, has a tensile modulus at 100° C.,defined as average of machine direction and transverse direction, withinthe range of about 7 to 70 MPa, as determinated according to ASTM D-638,preheated to a temperature of about 80°-130° C., at a draw ratio of lessthan about 1.8, defined as the ratio of the initial sheet thickness tothe minimum wall thickness of the thermoformed article, and at apressure differential of about 200-2000 kPa, in a mold having at leastone surface adjacent said sheet of average roughness R_(A), of about0.1-0.8 μm,the improvement, which produces a thermoformed article inwhich its essentially entire surface in contact with said mold surfacehas an R_(A) of about 0.1 to 0.8 μm and the concave optical imperfectionindex of said surface is less than about 6% of the surface area, asdetermined by image analysis using a microscope having a magnificationof 70 times, comprising the following steps: (A) removing air from thespace between said mold surface and said thermoplastic sheet through atleast one vent on the periphery of said mold surface to permit completecontact of said mold surface with said sheet, (B) while applying airpressure to the backside of said sheet, bringing said sheet, beforepoly(ethylene terephthalate) crystallinity exceeds about 15%, incomplete contact with said mold surface maintained at a temperature ofabout 130°-180° C., to permit the sheet polymer to completely replicatesaid mold surface, (C) maintaining said sheet in contact with the moldat a temperature of 130°-180° C. at least until the sheet polymerreaches a degree of crystallinity sufficient to permit demolding withoutdistortion, and (D) demolding the thermoformed article.
 2. Theimprovement of claim 1 wherein the draw ratio is less than about 1.3. 3.The improvement of claim 1 wherein the pressure differential (inkilopascals) is at least equal to 200×(tensile modulus of PET sheet at100° C., in megapascals)÷7.
 4. The improvement of claim 1 wherein themineral filler in glass fibers.
 5. The improvement of claim 4 whereinthe fiber average length to diameter ratio is at least
 10. 6. Theimprovement of claim 1 wherein the amount of filler is about 5-50% basedon the weight of the composition.
 7. The improvement of claim 6 whereinthe amount of filler is 10-35%.
 8. The improvement of claim 1 whereinthe PET sheet material also contains another polymeric material selectedfrom the group consisting of dipolymers of ethylene with an unsaturatedcarboxylic acid, terpolymers of ethylene with an unsaturated carboxylicacid and an ester of an unsaturated carobyxlic acid, and terpolymers ofethylene with an unsaturated carboxylic acid and a vinyl ester, theproportion of the carboxylic acid in the dipolymers being about 2-20weight percent, and the respective proportions of the carboxylic acidand of the ester in the terpolymers being about 1-30 weight percent ofeach; and the amount of the dipolymer or terpolymer being about 1-20%based on the weight of the reinforced polymer composition.
 9. Theimprovement of claim 8 wherein the carboxylic groups are partly orcompletely neutralized with a divalent metal ion.
 10. The improvement ofclaim 9 wherein the amount of the dipolymer or terpolymer is 3-10% basedon the weight of the reinforced polymer composition.
 11. The improvementof claim 1 wherein the mold temperature is about 140° C.
 12. In aprocess for thermoforming in a mold a first sheet of poly(ethyleneterephthalate) having a crystallinity of about 0-5% and a thickness ofabout 0.76-3.2 mm, which is reinforced with a mineral filler and, whenso reinforced, has a tensile modulus at 100° C., determined as anaverage of machine direction and transverse direction, within the rangeof about 7 to 70 MPa, as determined according to ASTM D-638 preheated toa temperature of about 80°-130° C., at a draw ratio of less than about1.8, defined as the ratio of the initial sheet thickness to the minimumwall thickness of the thermoformed article, the surface of the moldadjacent said sheet having an average roughness R_(A), of about 0.1-0.8μm, and a second, reinforcing, poly(ethylene terephthalate) backingsheet and bonding the second sheet to the first while both sheets are inthe mold,the improvement, which results in the first sheet having itsessentially entire unbonded surface of an R_(A) of about 0.1 to 0.8 μm,the concave optical imperfection index of said surface being less thanabout 6% of the surface area, as determined by image analysis using amicroscope having a magnification of 70 times, said improvementcomprising the following steps: (A) removing simultaneously orsequentially, beginning with the first sheet, air from the spacesbetween both sheets and their adjacent mold surfaces, maintained at atemperature of about 130°-180° C., through at least one vent on theperiphery of each mold surface and holding prior to the introduction ofair for about 1-4 seconds, (B) while applying air pressure between bothsheets, forming both sheets into their desired shapes while the firstsheet has a poly(ethylene terephthalate) crystallinity of at most about15%, to permit the first sheet polymer to completely replicate the moldsurface, (C) maintaining both sheets in contact with the hot moldsurfaces at least until the polymer in each sheet polymer reaches adegree of crystallinity sufficient to permit bonding without distortion,and (D) bonding the second sheet to the first with an adhesive, whilethe first sheet is in contact with the mold, (E) maintaining the firstsheet in contact with the mold until the bonded structure can bedemolded without distortion, and (F) demolding the bonded article. 13.The improvement of claim 12 wherein the first sheet is thermoformed at adraw ratio of at most 1.3.
 14. The improvement of claim 12 wherein thesecond sheet has the same composition as the first sheet.
 15. Theimprovement of claim 12 wherein the mineral filler is glass fibers. 16.The improvement of claim 15 wherein the average length to diameter ratiois at least
 10. 17. The improvement of claim 12 wherein the amount offiller is about 5-50% based on the weight of the composition.
 18. Theimprovement of claim 17 wherein the amount of filler is 10-35%.
 19. Theimprovement of claim 12 wherein the poly(ethylene terephthalate) sheetmaterial also contains another polymeric material selected from thegroup consisting of dipolymers of ethylene with an unsaturatedcarboxylic acid, terpolymers of ethylene with an unsaturated carboxylicacid and an ester of an unsaturated carboxylic acid, and terpolymers ofethylene with an unsaturated carboxylic acid and a vinyl ester, theproportion of the carboxylic acid in the dipolymers being about 2-20weight percent, and the respective proportions of the carboxylic acidand of the ester in the terpolymers being about 1-30 weight percent ofeach; and the amount of the dipolymer or terpolymer being about 1-20%based on the weight of the reinforced polymer composition.
 20. Theimprovement of claim 19, wherein the carboxylic groups are partially orcompletely neutralized with zinc, magnesium, or calcium ions.
 21. Theimprovement of claim 20 wherein the amount of the dipolymer orterpolymer is 3-10% based on the weight of the reinforced polymercomposition.
 22. The improvement of claim 12 wherein the moldtemperature is about 140° C.